The polymerization of ethylene homopolymers or copolymers is exothermic. One of the rate limiting factors in the industrial polymerization of polyethylene is the cooling rate for the reactors. In gas phase reactions the reactants, typically ethylene, one or more C3-8 alpha olefins and other feeds enter the reactor beneath a distributor plate having many small pores or holes in it. The gas flow up through the distributor plate into a bed of particulate matter, generally polymer particles enclosed by a vertical, generally cylindrical reactor of substantially the same diameter as the distributor plate. The upward flow rate and pressure of the feed gas supports and fluidizes the bed. The gas flows through the bed reacting with particulate catalyst in the bed, typically within growing polymer particles. Above the bed (reaction zone) the reactor diameter increases resulting in a pressure drop of the gas and hence the velocity as it leaves the reaction zone. In theory this the polymer particles fall back into the reaction zone and the gas then cycles through a heat exchanger and back to the reactor together with required additional feed.
In practice “fines”, very small particles of polymer, may be carried over from the reactor into the recycle loop passing through the cooler. The particles may contain active catalyst and tend to be deposited on the surfaces in the heat exchanger. Over time polymer builds up inside the heat exchanger and the pressure drop across the heat exchanger increases necessitating shutting down of the reactor and cleaning the heat exchanger.
Without wishing to be bound by theory there are also some theories of in-situ polymerization on the surface of the recycle line. This may also contribute to fouling or a combination of the fines and in-situ polymerization may cause fouling.
To increase the space time yield of the reactor (e.g. the cooling capacity) of the recycle stream, a condensable, non polymerizable hydrocarbon may be incorporated into the reactants. This hydrocarbon enters the reactor as a liquid phase and evaporates in the bed of polymer particles removing the heat of reaction. On passing through the heat exchanger the gas is condensed back to a liquid and then is returned to the reactor. This is generally referred to as condensing mode of operation or, depending on the amount of liquid, “super” condensing mode.
The first patents on condensing mode of operation are U.S. Pat. Nos. 4,543,399 issued Sep. 24, 1985 and 4,588,790 issued May 13, 1986 to Jenkins III et al., assigned to Union Carbide Corporation. The patents suggest that the liquid content in the recycle stream may be between about 2 and 20 weight % based on the weight of the stream. The 790 patent at column 7 lines 5 through 25 suggest that an excess of liquid in the recycle stream will help to prevent the build up of “mud” in sections of the recycle system where the flow rate is relatively low. The excess liquid may keep the system “washed clean”.
There is comparable teaching at column 9 lines 18 to 30 about “washing the system out” in U.S. Pat. Nos. 4,877,587 and 4,933,149 issued Oct. 31, 1989 and Jun. 12, 1990 to Rhee et al., assigned to Union Carbide Chemicals and Plastics Company.
The next improvement in the condensed mode is the so called super condensed mode of Exxon. The liquid level in the recycle stream is increased above 20 weight % up to 50 weight %. This is technology illustrated by U.S. Pat. Nos. 5,352,749 issued Oct. 4, 1994 to DeChellis et al.; 5,405,922 issued Apr. 11, 1995 to DeChellis et al.; 5,436,304 issued Jul. 25, 1995 to Griffin et al.; and U.S. Pat. No. 5,462,999 issued Oct. 31, 1995 to Griffin et al., all assigned to Exxon Chemical Patents Inc. Interestingly the 749, 304 and 999 patents all refer to “mud” as being a potential problem even though the patents claim a higher concentration of liquids in the recycle stream.
U.S. Pat. No. 6,800,692 issued Oct. 5, 2004 to Farley et al., assigned to ExxonMobil Chemical Patents Inc. discusses the issue of fouling of the reactor from column 22 line 57 through Column 23 line 34. The patent describes methods to measure fouling rate but does not contain any specific teaching to reduce fouling. The minimum acceptable fouling rate appears to be about 12% per month which would result in a fouling of about 30% to 40% in about 3 to 4 months, well above the fouling rate of the present invention.
All of the above art suggest increasing liquid levels in the recycle stream may reduce fouling but no further specific instructions are given about how to achieve the desired results. At best the patents set a course of experimentation for one skilled in the art.
U.S. Pat. No. 6,825,293 issued Nov. 30, 2004 to Goyal et al., assigned to NOVA Chemicals International S.A. teaches a process for controlling the properties of polymers prepared in the presence of a Ziegler Natta catalyst by controlling the feed rate of activator to the reactor based on the production rate of polymer. The patents teach the process may be used in conjunction with the above noted patents of Jenkins III, DeChellis and Griffin. However, there is no teaching of any reduction in fouling in the reactor.
U.S. Pat. No. 7,211,535 issued May 1, 2007 to Kelly et al., assigned to NOVA Chemicals Corporation and Ineos Europe Limited teaches catalyst of the type used in the present invention. The patent also teaches the catalyst may be used in conjunction with the teachings of Goyal and Jenkin III, DeChellis and Griffin. There is nothing noted in the application about a reduction of fouling nor are any specific operating conditions suggested which would reduce fouling.
Applicants have unexpectedly found that by continuously operating a reactor having a level of condensed liquids in the recycle stream greater than 13 weight % based on the weight of the recycle stream and using the catalyst of Kelly et al. with the controlled addition of alumina according to Goyal et al the reactor may be operated for not less than 24 months without having to shut down to clean the cycle gas cooler (heat exchanger). Assuming Farley is correct and it is necessary to shut down at 40% fouling this gives a monthly fouling rate (i.e. 40/24) of less than 1.7%.